The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the gating of NMR image data acquisition as a function of patient respiration.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins which may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The present invention will be described in detail with reference to a variant of the well known Fourier transform (FT) imaging technique, which is frequently referred to as "spin-warp". The spin-warp technique is discussed in an article entitled "Spin-Warp NMR Imaging and Applications to Human Whole-Body Imaging" by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (G.sub.y) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (G.sub.x) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse G.sub.y is incremented (.sup.21 G.sub.y) in the sequence of "views" that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
Most NMR scans currently used to produce high resolution 3D medical images, such as the image of coronary arteries, require many minutes to acquire the necessary data. Because of the long scan time, patient movement during the scan may be significant and can corrupt the reconstructed image with motion artifacts. There are also many types of patient motion such as respiratory motion, cardiac motion, blood flow, and peristalsis. There are many methods used to reduce or eliminate such motion artifacts including methods for reducing the motion (e.g. breath holding), methods for reducing the effects of motion (e.g. U.S. Pat. No. 20 4,663,591), and methods for correcting the acquired data for known motion (e.g. U.S. Pat. No. 5,200,700). In the case of respiratory motion, one of the best known methods for reducing motion artifacts is to gate the acquisition of data such that the views are acquired only during a preset portion of the respiratory cycle.
Prior respiratory gating methods employ a means for sensing patient respiration (e.g. U.S. Pat. No. 4,994,473) and producing a gating signal for the MRI system during a preset portion of the respiratory cycle. As long as the gating signal is produced, the MRI system acquires NMR data in the prescribed view order. During other parts of the respiratory cycle the gating signal is turned off and no data is acquired. As a result, when respiratory gating is used the scan time is increased significantly because data can only be acquired over a relatively short portion of each respiratory cycle.